Conventionally, heat energy recovery systems are known which recover waste heat of supercharged air to be supplied to an engine. For example, Japanese Unexamined Patent Publication No. 2011-74897 (hereinafter, referred to as “Patent Literature 1”) discloses a heat energy recovery system (fluid machine drive system) including an evaporator (second heat exchanger), a superheater (third heat exchanger), an expander for expanding working medium flown from the superheater, a fluid machine connected to the expander, a condenser for condensing the working medium flown from the expander, and a circulation pump for causing the working medium to flow from the condenser to the evaporator. The evaporator performs heat exchange between supercharged air discharged from an air compressor to be supplied to an engine and the working medium, to thereby evaporate the working medium. The superheater performs heat exchange between vapor flown from an exhaust gas boiler and the working medium flown from the evaporator to thereby heat the working medium.
Usually in such a system, the circulation amount of the working medium (rotation speed of the circulation pump) is controlled in such a way as to allow the degree of superheat of the working medium flowing into the expander to be within a specific range in order to prevent damage of the expander caused when the working fluid flows into the expander in liquid form, and to recover as much power as possible via the expander.
However, in the heat energy recovery system disclosed in Patent Literature 1, the amount of the heating medium (supercharged air) supplied to the evaporator and the amount of the heating medium (vapor) supplied to the superheater can change independently of each other. Consequently, when the rotation speed of the pump is controlled in such a way as to allow the degree of superheat of the working medium flowing into the expander to be within the specific range, the power recovery via the expander may become unstable.
For example, when the amount of the heating medium supplied to the superheater (the amount of heat put into the superheater) increases, the degree of superheat of the working medium flown from the superheater also increases. Accordingly, the rotation speed of the pump is increased in order to increase the circulation amount of the working medium. This, however, may make the pinch temperature (value obtained by subtracting a saturation temperature of the working medium from a temperature of the supercharged air) ΔT in the evaporator too small (see FIG. 5). In this case, the evaporation of the working medium in the evaporator becomes unstable, which makes the driving of the expander, i.e. the power recovery, unstable. Specifically, even though the frequency of the pump is constant, the suction pressure of the expander changes in various degrees and consequently, the output of the fluid machine changes. On the other hand, when the amount of the supercharged air to be supplied to the evaporator (the amount of heat put into the evaporator) decreases, the pinch temperature ΔT also decreases, which therefore makes the power recovery via the expander unstable in the same manner as in the above-described case. It should be noted that FIG. 5 is a graph showing relationships between the amount of exchanged heat and the temperature of heating mediums (supercharged air and vapor) and between the amount of exchanged heat and the temperature of the working medium in the evaporator.